1. Technical Field of the Invention
The present invention relates to an ignition device having improved ignition performance, which is used for an internal combustion engine.
2. Description of the Related Art
The internal combustion engine mounted to machines such as automobiles has been required to have improved fuel consumption and lean combustion in order to reduce the environmentally undesirable substances such as nitrogen oxides and carbon dioxide that are included in the exhaust gas.
An effective combustion means using a lean air-fuel mixture, on which the conventional spark-discharge type plug can not perform ignition, has been regarded as a desired internal combustion engine that can improve the combustion efficiency and can reduce these vehicle emissions with a mechanism of injecting plasma having high temperature and high pressure into the internal combustion engine.
Japanese Patent Application Laid-open Publication No. 2006-294257 discloses such an ignition device that comprises a housing for separating a chamber having an opening and a bottom face disposed opposite to the opening and having a circular shape in section; an external electrode provided at a surface of the housing which includes a hole for communicating the opening of the chamber with the outside; and a center electrode provided at a bottom face of the chamber.
Plasma is generated in the chamber by applying voltage between the center electrode and the external electrode to eject a plasma jet through the opening of the chamber. The cubic capacity of the chamber is made 10 mm3 or less, and the aspect ratio between the axial length and the inner diameter of the chamber is made two or more.
This ignition device allows the ejected plasma having high temperature and high pressure generated in the chamber to travel a long distance, and allows the fuel density of air-fuel mixture to be relatively high when using lean stratified combustion. Therefore, this ignition device was expected to improve the ignition performance of the lean fuel combustion.
However, this ignition device allows the plasma ejected in the internal combustion engine to maintain its high energy condition for a very short time only because of the high current applied in a discharge space for a very short time, 10 μsecond or less, caused by insulation breakdown in the discharge space by an application of high current.
Therefore, a relatively high energy, 200 mJ for example, was required to make flame kernels grow and spread in the air-fuel mixture to ignite. Further, advancement in the degree of density in the lean mixed gas was reaching its limit, though such high energy was provided. Moreover, the application of the high energy drastically wore electrodes, which limited the improvement of durability and reliability of the ignition device.
Lately, fluidization of the high gas in a combustion chamber has been progressed by improved swirl ratio or by generation of powerful tumble vortexes in the combustion chamber with use of a supercharger in order to properly mix fuel with compressed air.
For this reason, the conventional plasma ignition device could not easily ignite these combustion engines because flame kernels ejected in a combustion chamber are blown out by the powerful gas flow generated in the chamber, thereby losing energy before growing to a sufficiently large size needed for ignition.